Summary and Conclusions
The aim of this thesis was to fabricate low operating voltage and highly stable organic field-effect transistors (OFETs) using two different semiconducting materials as active channels. In order to achieve this, two strategies were followed. Firstly, we have considered gate dielectric materials with high-k dielectric constant for our study. Essentially, we have used bi-layer dielectric system considering different dielectric materials. The selection of the materials was based on the easy techniques available to grow the films and their different band energies. This enabled us to reduce the leakage current through the defects states. At the same time, the thicknesses of the dielectric layers were optimized to enhance the capacitance of the dielectric layer. In order to establish a better capacitance coupling between gate and organic semiconducting channel, the morphology of the dielectric layers were also studied to obtain smooth surface structure formation leading to minimum defect states. Secondly, we have studied the growth of organic active channel on top of the smooth dielectric surfaces to obtain the suitable growth parameters, which can enhance the performance of the devices. In one case, the organic active channel was in the form of single crystal wires like structures and in the other case, the active channel was in the form of thin film.
CoPc and PTCDI-Ph are the two molecules, which were used as the materials for active channel of the OFETs we fabricated in this thesis work. These two molecules are in the family of phthalocyanine and perylene derivatives, respectively. CoPc shows p-type semiconducting properties and PTCDI-Ph shows n-type semiconducting properties when devices are fabricated with these materials. To study the growth of thin films, we have used two solid substrate surfaces, such as SiO2 and mica (001). SiO2 is amorphous in nature, whereas, freshly cleaved mica (001) surfaces is atomically flat reconstructed surface.
Growth of CoPc thin film was studied on both the surfaces. However, PTCDI-Ph films were
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grown on different bi-layer dielectric systems. The bi-layer dielectric systems were fabricated using Al2O3 as one of the layers in combination with PVA, PMMA or any other metal oxides, such as TiOx, BaTiOx, SrTiOx and BaSrTiOx as second layer. These are the materials with high dielectric constant. Several OFETs were fabricated with these materials. The performances of the devices were studied optimizing different growth parameters. We successfully fabricated OFETs with high carrier mobility of the order 1.1 cm2/Vs with CoPc as active materials. We have demonstrated the performance of the OFETs with operating voltage as low as 1.2 V for the devices based on PTCDI-Ph. The stability of the devices was also checked in terms of stress bias effect, hysteresis and exposing to ambient for several days.
CoPc films were found not suitable for the fabrication of OFETs. We have observed roughening in CoPc film due to instability in the growth mechanism induced by local diffusion of the molecules. Charge transport through the organic channel is essentially driven by the hopping conduction from molecules to molecules which are connected by very week π−π interaction. The rough films as channel would further reduce the charge transport by introducing defects and grain boundaries in the film. The roughening in CoPc films were characterized by calculating different scaling exponents such as α, β and 1/z determined from the height fluctuations obtained from atomic force microscopy images of the surface morphologies. We have observed equal diffusion activation energy for lateral and vertical diffusion of the molecules. However, it was found that local surface diffusion, which exists even at 120 °C growth temperatures and it plays crucial role for roughening in CoPc film growth. The reported scaling exponents do not belong to the existing models, which hold well for inorganic film growth. Therefore, the roughening behavior observed in the present study appears to belong to a different universality class. The growth kinetics of the CoPc films were also studied and obtain the diffusion activation energy Ea = 0.35eV for the growth laterally as well as vertically. This represents the molecular translational and rotational barrier on the surfaces. However, we observed different activation energy for the growth of CoPc on mica (001) surfaces. The film morphology also remains rough in this case and found not suitable to fabricate OFETs based on these films. In order to improve the molecular packing within the OFET active channel, we used physical vapor deposition (PVD)
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technique exploiting the self-assembly of the CoPc molecules to grow CoPc micorstructures.
The molecules self-assembled to form CoPc single crystal micron size wires in PVD growth technique. We have achieved to grow more than 100 μm long CoPc wires after optimizing the growth conditions on different substrates. XRD and TEM measurements confirm the crystallinity of the wires. These wires were further used for the fabrication of OFETs.
We have used poly(vinyl-alcohol) (PVA)/Al2O3 system as bi-layer dielectric for the fabrication of CoPc wires based OFETs. Al2O3 layers were grown using anodization of Al films. We have grown a very smooth film of PVA due to the lower surface free energy of PVA (~0.045 J/m2) than Al2O3 (~1.7 J/m2). In this process, we achieved a better capacitive coupling between CoPc wires and gate through the bi-layer dielectric system. The thickness of the individual layers was optimized for minimum leakage current at the same time enhanced capacitance in the system. We have observed significant enhancement of carrier mobility, which is 1.11 cm2/Vs with on/off ratio ~104 and 20 V operating voltage. The observed carrier mobility of CoPc based OFETs is about one order of magnitude higher than the reported.
We have followed the same strategies to fabricate n-type OFETs based on PTCDI-Ph as active channel. The growth of PTCDI-Ph films on SiO2 and polymer dielectric surfaces have been carried out in order to obtain the suitable growth conditions to fabricate efficient devices. In this case, we have also considered using bi-layer dielectric consisting layers of poly (methyl methacrylate) (PMMA) and Al2O3. The field-effect carrier mobility obtained from the devices designed at 90 °C substrate temperature showed the highest value of 0.02 cm2/Vs. This has been attributed to the formation of smooth and uniform films of PTCDI-Ph on PMMA surfaces. In this process, we were able to enhance the carrier mobility. However, the operating voltage for these OFETs was about 30 V, which is still very high.
In order to fabricate the OFETs based on PTCDI-Ph as active channel with lower operating voltage, we have replaced the PMMA layers by the materials with higher dielectric constant. We have synthesized high-k metal-oxide gate dielectric sols like TiO2, BaTiO3, SrTiO3 and BaSrTiO3. The device parameters were optimized to obtain lower leakage current and formation of smooth dielectric surfaces. We have successfully achieved low operating
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voltage OFETs by introducing solution-processed high-k bi-layer systems as the gate dielectric. The bilayer dielectric systems exhibits very smooth surfaces with rms roughness below 1 nm, high capacitance about 65 nF/cm2 and low leakage current. PTCDI-Ph based OFETs exhibit an electron mobility about 0.04 cm2/V-s and an operating voltage as low as 1.2 V with threshold voltage ranging from 0.3-0.5 V. The obtained results for all the transistors are much better than the transistors fabricated with the traditional SiO2 dielectric.
Most importantly, we demonstrated air stable n-type OFETs since all the measurements were performed under ambient condition. The observed parameters are highly reproducible and reliable.
In order to study the stability under bias stress and exposing into air, we have performed measurement on one of the best devices fabricated above. The stability of the devices was studied through hysteresis and bias-stress response. Negligible hysteresis in transfer characteristics have been observed for these devices in compare to the devices fabricated with SiO2 dielectric. In that case, we observed huge hysteresis (corresponding threshold shift is > 4V). The air stability of the devices was studied for 150 days. We have observed slow decay process of carrier mobility to 0.02 cm2/Vs in 68 days. However, these devices were found to be quite stable without showing any further decay in carrier mobility up to 150 days.
This result conforms that the high reliability of the OFETs with high stability under bias stress. The periodic application of bias stress also confirms that the devices are extremely stable.
In this thesis, we have successfully fabricated OFETs based on CoPc and PTCDI-Ph molecules with better performances. Some of the open questions could be tackled within this work, but at the same time, new unsolved questions arose, which pave the way to interesting studies in the near future. In the following, some of the most intriguing issues are listed.
We have observed roughening in CoPc thin film growth even at elevated substrate temperature. Our experimental results confirmed that the local diffusion of the molecules is apparently reason for this roughening mechanism. The observed exponents do not satisfy any of the existing growth model for inorganic film growth. It would be interesting to propose a
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simple solid on solid model including local diffusion of the molecules to get insight of the mechanism.
We have observed long CoPc structures formation on mica (001) surfaces. The effect of surface reconstruction on the growth of CoPc films on mica (001) surfaces needs more involved experimental study. X-ray surface diffraction study could be one of the appropriate studies to probe the molecular arrangement at the interfaces. In order to successfully implementation of the structures into electronic device fabrication, it would be very important to have the control on the growth of these structures with specific directionality.
For further comprehensive description of the involved interactions might also be employed.
We have successfully fabricated n-type air stable PTCDI-Ph based OFETs with bi-layer dielectric system for low voltage operation. The stability of the devices is also very promising. The interaction of polymer or high-k dielectric surfaces with small molecules is more complex than the inorganic surfaces like SiO2. It would be interesting to understand the molecular interactions, which lead to form different surface structures. The complete understanding of the growth, could guide to fabricate devices with enhanced performances.
We have observed an anomalous increase in current during bias stress measurement.
More involved experiments require to understanding this effect. Though, proton migration model is proposed to explain this effect for p-type OFETs, but the mechanism could be different for n-type OFETs. Nevertheless, the interaction of water with STO surface is more complex. Moreover, the partial current flow through the STO layer cannot be ruled out. More controlled experiments are required to be short out these issues.
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List of Publications
1) Murali Gedda, Nimmakayala V. V. Subbarao and Dipak K. Goswami. “High carrier mobility of CoPc wires based field effect transistors using bi-layer gate dielectric” “AIP Adv.”, 3, (2013) 112123-112127; doi: 10.1063/1.4834355.
2) Murali Gedda, Nimmakayala V. V. Subbarao and Dipak K. Goswami. “Local Diffusion Induced Roughening in CoPc Thin Film Growth”, “ACS Langmuir”, 30, (2014), 8735- 8740, doi: 10.1021/la502108a.
3) Murali Gedda, Nimmakayala V. V. Subbarao and Dipak K.Goswami “Growth mechanism of Cobalt(II) Phthalocyanine(CoPc) thin films on SiO2 and muscovite substrates” “AIP Conf. Proc”. 1576, (2014), 152-154; doi: 10.1063/1.4862007.
4) Nimmakayala V. V. Subbarao, Murali Gedda, V. Suresh, Parameswar K. Iyer and Dipak K. Goswami. “Effect of hybrid gate dielectric on perylenediimide based organic field effect transistors”, “Phys. Status Solidi A”, 1–9 (2014); doi: 10.1002/pssa.201431304.
5) Nimmakayala V. V. Subbarao, Murali Gedda, Parameswar K. Iyer and Dipak K.
Goswami. “Enhanced environmental stability induced by effective polarization of a polar dielectric layer in tri-layer dielectric system of organic field-effect transistors: a quantitative study” “ACS AMI”, 2015; doi: 10.1021/am507636k.
6) Nimmakayala V. V. Subbarao, Murali Gedda, V. Suresh, D. Anamika, Parameswar K.
Iyer and Dipak K. Goswami. “Growth and Characterization of N, N′-Dioctadecyl -1, 7- Dibromo-3, 4, 9, 10-Perylenetetracarboxylic-Diimide Micron/Nano Wires for Organic Field Effect Transistors” “AIP Conf. Proc”., 1576, (2014); 42-45, doi:
10.1063/1.4861975.
7) Murali Gedda, Arindam Pal, Nimmakayala V. V. Subbarao, M. Sharma, Parameswar K.
Iyer and Dipak K. Goswami. “Effect of substrate temperature on the growth of PTCDI-Ph films on a hybrid dielectric materials for organic field-effect transistors”, “Organic Electronics”, 2014 (Submitted).
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8) Murali Gedda, Nimmakayala V. V. Subbarao and Dipak K. Goswami, “PTCDI-Ph based low-operating voltage organic field effect transistors with solution processable gate dielectrics”, “ACS AMI”, 2015 (Submitted).
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Conference Presentations
1) Murali Gedda, Arindam Pal, P. Anand Kumar, and D.K.Goswami “Kinetics Of Flat Top Organic Nanostructure Growth”, Second International Conference on Advanced Nanomaterials and Nanotechnology (ICANN-2011), Dec 8-10, 2011, IIT Guwahati.
India.
2) Murali Gedda, Arindam Pal, P. Anand Kumar, and D.K.Goswami “Kinetics Of Flat Top Organic Nanostructure Growth”, International Conference on Nano Science and Technology (ICONSAT 2012), January 20-23, 2012, ARCI, Hyderabad. India.
3) Murali Gedda, Arindam Pal and D.K.Goswami “Study of Growth of Organic Thin Films by X-ray Reflectivity Measurements” “12th International Conference on Surface X-ray and Neutron Scattering (SXNS-12)” Saha Institute of Nuclear Physics, Kolkata, 25th to 28th July, 2012, India.
4) Murali Gedda, Arindam Pal, V. Suresh, P.K Iyer and D.K. Goswami, "Effect of Temperature on the Molecular Arrangement of Organic Nano-structure Growth” 4th International conference on Advanced Nanomaterials (ANM 2012) IIT Madras, Chennai, October 17 –19, 2012, India.
5) Murali Gedda, Nimmakayala V. V. Subbarao and Dipak K. Goswami, “Growth kinetics of Cobalt Phthalocyanine (CoPc) Thin Films Grown by Molecular Beam Deposition Technique” 2nd International conference on Optoelectronic Materials and Thin films for Advanced Technology (OMTAT 2013), CUSAT, Kochi January 3- 5, 2013, India.
6) Murali Gedda, Nimmakayala V. V. Subbarao and Dipak K. Goswami “Catalyst Directed Growth of Cobalt (II) Phthalocyanine Organic nanostructures”, Third International Conference on Advanced Nanomaterials and Nanotechnology (ICANN-2013), 2013, Center for Nanotechnology, IIT Guwahati, India.
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7) Murali Gedda, Nimmakayala V. V. Subbarao and Dipak K. Goswami “Solution- processed metal-oxides as gate dielectrics for low-operating voltage of organic field- effect transistors”, 3rd International Conference on Physics at Surface and Interfaces (PSI 2014), Feb 24-28, 2014, Puri, India.
Schools & Workshops Attended
1) “International School on Nanoscience with X-Ray and Neutron Sources”, organized by Saha Institute of Nuclear Physics, Kolkata, from 23th to 24thJuly, 2012, India.
2) “INUP Familiarization Workshop” especially for the North East at IIT-Guwahati”
during 28-29 September 2012, India.
3) “INUP Hands on training” organized by CeNSE, IISc Bangalore, from 24th Jun to 03rd July, 2014, India.